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Report:Groundwater-quality Monitoring—July 27,2004: 7500 West Eleventh Street,Tracy, CA. Page 5 beneath the ground surface and to be some 100 ft. thick. The total thickness of the underlying Lower Tulare Formation is not well documented; however, estimates suggest that it ranges in thickness from 300 ft. to greater than 1,400 ft. Figures 4-9 are hydrostratigraphic longitudinal and cross-sections drawn along section lines A-A' through F-F', the locations of which are shown on Figure 2. To reduce the complexity of the stratigraphy of the Navarra Site so that it is tractable to practical r= interpretation, the sections shown. on Figures 3-8 were developed by dividing the sediments into general classes based on their hydraulic conductivity and cation exchange capacity, which are the soil characteristics of primary relevance to contaminant transport analysis. In the cross-sections, the permeable gravels, sands and silts have been segregated from the relatively less permeable clays and silty clays. :: The depth to groundwater beneath the site varies seasonally between 7 and i 1 ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosic branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street site. However, locally, the shallow groundwater gradient tends to follow the topography, which, at the subject property, slopes gently to the north-northeast. The local direction of F-71 1 groundwater flow is also affected by the local sedimentary geology, particularly where continuous or semi-continuous sand strata provide channels for subsurface flow through less permeable facies. E- Based on pump tests that SJC conducted in similar strata at another location in Tracy, and from the observed rate of migration of methyl tertiary butyl ether (MTBE) through the subsurface, it is estimated that the sands beneath the site have a mean horizontal f hydraulic conductivity of approximately 1.5 to 10"2 cm/sec (The San Joaquin Company Inc. 1994, 2002c). The vertical permeability of the clay strata that separate the several aquifers present at depths in the range from ground surface to 50 ft. BGS was assessed by L-J a constant-head permeability test that was conducted on a sample of clayey soil recovered in April 2004 from a depth of 7.5 ft BGS. The vertical permeability of that sample was r> found to be 10.8 x 10"7 cm/sec. 1.5 Distribution of Hydrocarbons in the Subsurface The several stages of tank removal, excavation and removal of contaminated soil and groundwater, site characterization and groundwater-quality monitoring that have been 1.,-4 conducted at the 7500 West Eleventh Street site since December 1998 have permitted synthesis of historic, geologic, hydrostratigraphic and geo-chemical data. That synthesis has resulted in the following interpretation of the distribution of hydrocarbons in the k: subsurface. 1.5.1 Primary Plume y The site is affected by a primary plume of diesel and gasoline that has affected both soil and groundwater. As is shown on Figures 2 through 8, it emanates from the area where SJC